1. Technical Field
This invention relates to a laser-based optical inspection system for inspecting products for flaws, and more particularly to a laser-based optical inspection system that detects and measures surface imperfections, including warpage, in optical information storage disks.
2. Discussion of Related Art
With all optical disks used for information storage, including analog and digital audio disks, compact digital disks (i.e., CDs) and digital video disks (DVD), it is important that these disks have a high degree of flatness. Because these types of disks are normally made of plastic, they can warp if subjected to sufficiently high temperatures or other thermal or mechanical stresses during the manufacturing process or afterwards. An audio record that has been too severely warped cannot be played. The same is true for optical disks. As technology has evolved, the storage capacity of optical disks has increased so that even slight warpage can impact the performance of the disks. A typical compact disk has a digital storage capacity of approximately 600 to 700 megabytes of information. Similarly, a new DVD has an even greater storage capacity (in the range of 4 to 16 gigabytes). As a result, optical components, such as the laser diodes and focusing systems used to read these disks, are much more precise and less tolerant of surface imperfections on the disks and of warped disk surfaces.
Due to the lack of equipment for quickly measuring warpage of optical disks, or quickly and inexpensively finding blemishes that would result in recording and playback errors, manufacturers often record information onto a disk, only to find out later that the disk was warped or has a surface blemish. Presently, a common quality control inspection approach for detecting the warpage of these disks is to play back the disk at a relatively high rate of speed. However, due to the immense storage volume of CD's and DVD's, the play back time (even on a sampling basis) for such disks is considerable, and cannot keep pace with the rest of the manufacturing processes.
Alternatively, visual detection methods using conventional camera systems are also utilized during quality control inspections by disk manufacturers. This method is limited to detecting imperfections on the surface of the disk, and although the detection resolution of this method is sufficient for inspecting audio disks, it is inadequate and/or too time-consuming for optical disks, particularly DVDs.
Currently, manufacturers of information storage disks require a low-cost, high-speed approach which can make high resolution measurements to test if a disk is flat within a desired degree of tolerance, and as to whether or not it has any unacceptable blemishes. In other words, manufacturers would like to be able to quickly and automatically inspect disks that are being fabricated in an automated line and reject those which have an excessive degree of tilt or warpage or which have unacceptable surface defects. At present, as far as we know, the industry is unaware of any simple, fast and practical means to detect warpage in an automated and speedy fashion.
Therefore, it is desirable to provide a high resolution and high speed optical inspection system for detecting and measuring warpage and other surface imperfections in optical information storage disks, especially DVDs. Since existing information storage disks are manufactured in a multiple step process, wherein laminated stacks of material are built up to produce a final disk product, it is also desirable to provide optical inspection equipment which can, if desired, perform inspection after each layer of the disk has been constructed and thus the present invention must operate at very high speed. Further, it is an object of the present invention to provide equipment used to detect disk warpage that is relatively compact and low cost, so that it does not add considerably to the space required for or the price of existing automated disk manufacturing facilities.